Fringe projection profilometry (FPP), as a three-dimensional measurement technology based on fringe projection, has been widely studied and applied in recent years, because it has advantages of high accuracy and high speed, and is less sensitive to ambient light. The FPP also has limitations as a three-dimensional measurement method based on active light projection. The most obvious one of the limitations is that an industrial camera used in a measurement system produces various noises during practical measurements. Such noises may reduce the signal-to-noise ratio of a collected image and affect the quality of a fringe image, and thus the quality of a calculated phase and the accuracy of the final three-dimensional reconstruction. This phenomenon becomes relatively serious especially when the measured object has low surface reflectance.
For an object with complicated surface texture, especially, with dark texture, the signal-to-noise ratio of images collected by the FPP system is relatively low, and as a result, the quality of the phase information obtained from dark textures in the captured image is poor. To solve such a problem, most existing solutions either increase the size of the camera aperture, the shutter speed, and the camera gain; or entirely enhance the intensity of projected light of a projector, such that the part of captured images with dark texture is sufficiently exposed. This method can effectively improve phase information of dark textures. However, the increased camera aperture size, shutter speed, and camera gain may saturate part of the image, that is, the intensity value of the images reaches 255 (for an industrial camera with an 8-digit image format). Especially, when the surface texture of an object is rather dark, this method causes severe image saturation. In addition, this method usually needs to measure an object many times, and then combine different measurement results into one final result. An operation in the whole measurement process is rather complicated and it is difficult to quantify the adjustment of camera parameters.
The dark surface texture affects the quality of the fringe image in the following two aspects: sampling performance of the camera and random noises. An influence of the sampling performance of the camera on the quality of the fringe image may be relieved by using a digital camera with larger image depth or using the foregoing methods of repeated exposure and adjustment of projector brightness. The random noises need to be compensated by additionally setting an algorithm, which is the primary objective of this invention.